Auto select account when there's only one

[dic1911]

Is your feature request related to a problem? Please describe.
There are two places where I find this applicable:

• When I try to use cloud backup feature in LINE(jp.naver.android.line) the account selector just keeps pop up again and again
• When Blue Archive(com.YostarJP.BlueArchive) is opened it'd show a black screen for a short moment, then a "Play Games"-like account selector would pop up

Describe the solution you'd like
Add a toggle in microG settings to auto select the account and prevent the selector popup from being shown.

Describe alternatives you've considered
N/A

Additional context
Add any other context or screenshots about the feature request here.